Calibrating the perceived strength of evidence of forensic testimony statements

Pattern comparison disciplines use categorical statements to express conclusions. We measured the strength of evidence for six different scales as perceived by members of the general public and fingerprint examiners. The statements came from different types of scales, and included categorical conclusions, likelihoods, strength of support statements, and random match probabilities. We used an online interface that required participants to first correctly sort the statements in a given conclusion scale, and then place each statement on a single evidence axis that ranged from most support imaginable for same source to most support imaginable for different sources. We analyzed the data using both the raw values and a Thurstone-Mosteller model based on ordinal values. We found systematic differences between examiners and members of the general public, such that examiners distinguished between Identification and Extremely Strong Support for Common Source, while members of the general public did not. Statements that included numerical values tended to be placed lower than categorical conclusions, and members of the general public tended to place the highest categorical conclusion in each scale at the very top of the evidence axis. The results suggest that laypersons can distinguish between statements meant to represent moderate vs strong evidence, but tend to place categorical conclusions above statements that involve numerical values.